BoilerFaultCodesGlow-worm F21 Fault Code
The Glow-worm F21 fault code means: your boiler has lost water pressure and has shut down as a safety precaution to prevent damage.. It appears on 1 Glow-worm model in our database.
Technical description: Low water pressure
What causes the Glow-worm F21 fault?
This fault occurs when the water level inside your central heating system drops below the minimum level required for the boiler to operate safely. The most common reasons are a slow leak from a radiator valve or pipework, or the air pressure inside the internal expansion vessel needs topping up.
This fault often appears when you first turn the heating back on for autumn, as internal components can stick or minor leaks become apparent after a summer of inactivity.
DIY fix possible
Locate the filling loop (usually two silver braided hoses) beneath the boiler.Gradually open the small black taps until you hear water entering the system.Watch the pressure gauge and close the taps once the needle reaches 1.5 bar.
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Gas Safe Register →Glow-worm models that show the F21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F21 fault code information on this page is sourced and cross-checked against the official Glow-worm service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026